The present invention relates to an external storage subsystem, and more particularly to a technology effective for an improvement of reliability of an external storage subsystem having a cache function.
For example, in a magnetic disk subsystem used as an external storage in a general purpose computer system, a well-known cache memory comprising a semiconductor memory is interleaved at a portion of a disk controller to avoid as much as possible the reduction of a data transfer rate due to a mechanical factor such as a rotational delay time or a latency time in a magnetic disk drive.
A cache structure in such a disk controller is discussed in "A Multiport Page-Memory Architecture and A Multiport Disk-Cache System" New Generation Computing 2 (1984) 241-260 OHMSHA, LTD. and Springer-Verlag, in which it is proposed to improve an access performance to the cache by dividing it into a plurality of memory banks. Further, a switching network, called an interconnection network, is proposed as a system for coupling the memory banks and a channel or a disk controller.
The conventional technology described above intends to improve the cache performance by providing a plurality of memory banks and the switching network. The network system called the interconnection network provides the bus structure in the disk controller. However, the switching network system is imparted with a hardware restriction when a data bus configuration for exchanging data is to be constructed by connecting a plurality of memory banks and a plurality of channel units or a plurality of control units.
Typically such a configuration does not take into account the nature of the cache unit comprising the memory banks.